Topic 3: Second and Third Law of Thermodynamics

Question 1

what is the 2nd law of thermodynamics

Answer 1

in any spontaneous process, there is always an increase in the entropy of the universe

total energy of universe is constant, but entropy is increasing

Question 2

what is carnot cycle and what are the 4 steps

Answer 2

theoretical model that describes the most efficient heat engine > understand how real heat converted to work energy

1. isothermal expansion
2. adiabatic expansion
3. isothermal compression
4. adiabatic compression

Question 3

what happens during isothermal expansion in carnot cycle

Answer 3

engine takes in heat (Q1) from hot source > immediate converted to work by gas expanding, pushing piston outward

since temp stays constant > absorbed heat fully converted to work

Question 4

what happens during adiabatic expansion in carnot cycle

Answer 4

gas continues expanding, pushing piston further, but no heat is added or removed > gas loses internal energy > cool down

Question 5

what happens in isothermal compression in carnot cycle

Answer 5

now surroundings push piston back > compressing gas > makes gas give up heat (Qc) to cold sink

since temp remains constant > released heat prevents temp change

Question 6

what happens during adiabatic compression in carnot cycle

Answer 6

gas is further compressed but no heat is lost or gained > compression causes gas temp to rise back to original temp > ready for next cycle

Question 7

what happens to carnot cycle when there is large temperature difference between system and heat source

Answer 7

uncontrolled and rapid heat flow > reaction becomes irreversible

instead of converting heat to work efficiently, portion of heat is wasted just to warm up the system > less heat available for conversion into useful work > efficiency lower than ideal carnot efficiency

Question 8

what happens if system pressure is much more than external pressure (constant external pressure expansion)

Answer 8

gas in system will expand in uncontrolled and rapid manner > reaction becomes irreversible

some of energy is wasted in chaotic molecular motion instead of performing useful work on surroundings > less work done > efficiency lower than ideal carnot efficiency

Question 9

why is entropy of irreversible process always higher than that of reversible

Answer 9

irreversible produce additional entropy

Question 10

what is the third law of thermodynamics

Answer 10

the entropy of a perfect crystal at 0K is zero

the entropy of a substance increases with temperature